Modern suites of well logging measurements are used to predict porosity and fluid saturations of reservoir rocks surrounding a borehole. Porosity and fluid saturations are useful for accurate reserve estimation and identification of potential hydrocarbon bearing zones. More accurate porosities and fluid saturations may be predicted if detailed and accurate mineralogical information is available. Mineralogical data provide more accurate characterization of logging tool responses and, as a result, lead to improved log interpretations. Knowledge of the clay mineral types present in reservoir rocks and their volumes is an indicator of reservoir quality and is also used in the selection of hydraulic fracturing, completion, and stimulation fluids.
Elemental spectroscopy logging tools provide elemental compositions of reservoir rocks (e.g., Si, Al, Ca, Mg, K, Fe, S, etc.) derived from capture and inelastic neutron gamma ray spectroscopy. The elemental compositions are given as the weight fractions of the individual elements present in the rock matrix. They are used to predict mineralogy and rock properties such as grain density. The inversion of elemental composition to predict accurate mineralogy is a complex issue in reservoir characterization. The complexity arises because of the large number of minerals that are commonly found in reservoir rocks and the variability of the compositions of these minerals. Moreover, the mineralogy inversion problem may be complicated by the fact that many of the measured elements are common to different minerals. Thus, there exists a degree of non-uniqueness in the reconstruction of mineralogy from elemental composition data.
Because of the complexity of the mathematical relationship between elemental composition and mineralogy it is difficult to derive accurate forward models that predict mineralogy from rock chemistry. This is also true for most other reservoir characterization issues for which idealized forward models do not accurately account for the behavior of complex reservoir rocks and fluids.